This invention relates to block polymers of conjugated dienes which contain hydroxyl groups, including highly reactive primary hydroxyl groups, in the diene blocks. The invention also relates to a process for making such polymers from epoxidized diene block polymers.
Conventional A-B-A block copolymers are known to be useful in coatings, sealants, adhesives and modified asphalts but their usefulness in such products is not as great as it could be if these polymers did not suffer from deficiencies in certain physical properties. For instance, U.S. Pat. No. 3,792,005 disclosed that coatings can be made using block polymers of A-B-A type where A is a monovinyl aromatic polymer block, usually polystyrene (S), and B is a rubber polymer block, usually hydrogenated polybutadiene (EB) or hydrogenated polyisoprene (EP). These polymers could be especially useful in elastomeric coatings because they can be formulated to have good flexibility and therefore, will not crack during thermal cycling, an important requirement for roof coatings, for example, or during metal forming, where the coating becomes stretched as the metal is bent. However, coatings based on conventional A-B-A type block copolymers are deficient in that they lack strong adhesion and in applications in which the coating will contact organic liquids such as gasoline, or high temperatures, the coatings merely dissolve or melt off of the substrate.
It would be advantageous to provide block polymers of this type which had enhanced physical properties. By functionalizing these conventional block polymers, they can be crosslinked to give polyurethane structures which will have better physical properties and thus be more useful than the conventional block polymers in many coatings, sealants, adhesives and modified asphalts.
The known curable or crosslinked coatings which are based on vinyl aromatics and conjugated diolefins do not have particularly good long term heat, weather and ultraviolet stability due to the need to utilize unhydrogenated polymers (it is difficult to add functional groups without using unhydrogenated polymers). Hydrogenation is known to improve long term heat, weather and ultraviolet stability, but it removes the double bonds which are needed to add functional groups which can be crosslinked.
It is an object of the present invention to provide a block polymer which can be modified so that it is substantially free of residual olefinic double bonds and can be crosslinked by reaction with amino resins and isocyanates. Further, it is an object of this invention to provide a coating composition which is based on this crosslinkable block polymer.
It is also an object of the present invention to provide a process for making such hydroxyl functional block polymers which achieves high conversion to a well defined product under mild conditions. In the past, protic acid and Lewis acid catalysts have been used to react epoxidized polymers such as disclosed in U.S. Pat. No. 5,015,697. However, protic acid catalysts generally require long reaction times and many Lewis acid catalysts lead to side reactions such as ketone or aldehyde formation. Gel formation has also been a problem in these systems. It would be advantageous to provide a process for making hydroxyl functional derivatives of epoxidized diene block polymers under mild conditions which avoid these problems. The present invention provides such a means by opening hindered epoxy rings to introduce hydroxyl functionality, including primary hydroxyls, to the polymer backbone.
Epoxidation has proven to be widely applicable to block copolymers of vinyl aromatic hydrocarbons and conjugated dienes, including partially unsaturated versions thereof, as discussed in U.S. Pat. No. 3,555,112 and U.S. Pat. No. 3,607,982. Unfortunately, the resulting epoxides of such polymers do not readily participate in reactions used to cure most coatings. Conversion to hydroxyl functionality would be highly desirable but, as discussed above, there are disadvantages to the currently known processes.